1. Field of the Invention
The present invention relates to a thin film transistor ("TFT") and a method for fabricating the same and, more particularly, to a TFT and a method for fabricating the same in which the resistance of the source and the drain is reduced.
2. Discussion of Prior Art
A Liquid Crystal Display ("LCD") has a plurality of pixel cells which include a switching device as a driving element and a pixel electrode as its basic units which are arranged in a matrix pattern. The switching device is a TFT including a gate region, a source region and a drain region.
TFTs are classified as top gate type TFTs in which a gate electrode is formed over an active region and bottom gate type TFTs in which a gate electrode is formed below an active region according to the relative position of the active region and the gate.
FIG. 1 is a cross-sectional view of a bottom gate type TFT of a conventional device.
A gate electrode 13 consisting of a conductive material, such as Al is formed on an insulating substrate 11. A gate insulating layer 15 consisting of an insulating material, such as silicon oxide or silicon nitride is formed on the substrate 11 so as to cover the gate electrode 13. An active layer 17 consisting of amorphous silicon is formed on the gate insulating layer 15 over the gate electrode 13. An ohmic contact layer 19 consisting of heavily-impurity-doped-amorphous silicon is formed on the active layer 17 so as not to overlap the gate electrode 13.
A source electrode 21 and a drain electrode 22 are formed on the ohmic contact layer 19. The source and drain electrodes 21 and 22 are formed by a refractory metal material, such as Cr or Ta. A passivation layer 23 consisting of an insulating material, such as silicon oxide or silicon nitride is formed on the entire surface of the resultant structure. A contact hole 25 is formed in the passivation layer 23 to expose a portion of the drain electrode 22. A pixel electrode 27 is formed on the passivation layer 23 and is electrically connected to the exposed drain electrode 22. The pixel electrode 27 is formed by a transparent material, such as Indium Tin Oxide ("ITO").
FIG. 2 is a cross-sectional view of a top gate type TFT according to a conventional device.
An active layer 33 consisting of amorphous silicon is formed on an insulating substrate 31. A gate insulating layer 35 and a gate electrode 37 are formed sequentially on the active layer 33. A source region 41 and a drain region 42 are formed in the active layer 33. The source and drain regions 41 and 42 are heavily doped by N-type impurities or P-type impurities.
An insulating interlayer 35 consisting of an insulating material, such as silicon oxide or silicon nitride is formed on the entire surface of the resultant structure. First contact holes are formed in the insulating interlayer 35 to expose a portion of the source region 41 and a portion of the drain region 42. A source electrode 44 connected to the exposed source region 41 and a drain electrode 45 connected to the exposed drain region 42 are formed on the insulating interlayer 35. The source and drain electrodes 44 and 45 are formed from a refractory metal material, such as Cr or Ta.
A passivation layer 47 consisting of an insulating material, such as silicon oxide is formed on the insulating interlayer 39 covering the source and drain regions 41 and 42. A second hole 49 is formed in the passivation layer 47 to expose a portion of the drain electrode 45. A pixel electrode 51 is formed on the passivation layer 47 and electrically connected to the exposed drain electrode 45. The pixel electrode 51 is formed of a transparent material, such as ITO.
As described above, the source and drain electrodes are formed by a refractory metal material, such as Cr or Ta in the bottom gate type TFT and the top gate type TFT according to the prior art devices. When forming the source and drain electrodes using a refractory metal material, such as Cr or Ta, the contact spiking generated in the contact surface of the amorphous silicon layer and the hillock generated in the surface may be prevented.
However, the refractory metal material makes the resistance of the switching device increase, thereby causing a reduction in the switching speed.